This is normal, it's suppose to show that the taskbar is slightly curve rather than flat. Most people only notice it if they use a contrasting background. And yes, I had it too; just moved to windows 7.

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When the taskbar is moved to the side or top of your desktop, does it still have that line?

well yes if i move the taskbar left,right or up the line is gonebut if i put it back the line is still there since i reinstalled vista i havent patched yet with vista glazz to use themesand i dont have the line...

Well there is an alternative of vista glazz try replasing the dll with alredy patched one to support custom themes or try with tuneup utilities it has trial 30 days and with this program you can also apply custom themes... and if the problem is still there maybe try different video drivers or update to Service Pack 1